The present invention relates to a rim for a bicycle spoked wheel, particularly a bicycle rear wheel, of the type comprising a hollow profile having a radially inner wall, a radially outer wall, and two side portions connecting said walls with each other, said radially inner wall having a plurality of spoke anchoring spots, said anchoring spots being arranged on a plane offset with respect to a median plane orthogonal to the rim axis and equally spaced apart from the side portions thereof.
A rim of the above indicated type is described and illustrated for example in European patent EP-B-0 494 277, in corresponding U.S. Pat. No. 5,228,756 and in German utility model G 89 12 606.8.
FIG. 1 of the annexed drawings shows a partial cross-section of a bicycle rear wheel made according to the prior art. Reference numeral 1 generally designates the rim, constituted by a hollow metal profile, having a radially inner wall 2, a radially outer wall 3 and a pair of side portions 4, 5. Reference numeral 6 generally designates the hub of the wheel (shown only partially) having an axis 7. Since the hub 6 is a hub of a rear wheel, it is adapted to carry a number of sprockets able to selectively engage the bicycle chain. According to the conventional art, the sprocket cluster (not shown) is mounted on a freewheel unit 8 arranged on one end of the wheel hub 6. The hub 6 also has a pair of circumferential flanges 9, 10, one arranged adjacent to the freewheel unit 8, the other flange being adjacent to the opposite end of the hub 6. the two flanges 9, 10 are used for connection of the ends of two series of spokes 11, 12 (shown only diagrammatically in the annexed drawings) which are arranged substantially on two conical surfaces having a common base in a plane 13 orthogonal to the axis 7 of the hub 6 and rim 3, and apices located on opposite sides with respect to this plane 13. This arrangement is due to that the outer ends of both series of spokes 11, 12 are connected at the inner wall 2 of rim 1 at a plurality of anchoring spots 14 defining a common median plane which is coincident with the above mentioned plane 13 orthogonal to the axis 7. Since the plane 13 of the anchoring spots 14 for the spokes is substantially equally spaced from the ends of hub 6, the presence of the freewheel unit 8 causes a positioning of the anchoring flanges 9, 10 which is not symmetrical with respect to the above mentioned plane 13. As a consequence of this, the angles .alpha. and .beta. of the two spoke series 11, 12 on the left and right sides of the bicycle wheel are differ greatly from each other. This condition implies a great difference in the strains induced in the spokes on the right and left sides of the bicycle wheel during the assembling operations of the wheel, with detrimental consequences on the strength of the structure and the ability of the wheel to keep its operative characteristics with the time.
In order to decrease the above mentioned drawback, it is necessary to render the two angles .alpha. and .beta. less different as possible. This result is achieved in the known rims indicated at the beginning of the present description by offsetting the median plane of the anchoring spots 14 of the spokes with respect to the median plane 13 of the rim. A known solution of this type is shown in FIG. 2 of the annexed drawings. As shown, in this figure the two angles are .alpha.' and .beta.', less different from each other with respect to the case of FIG. 1, due to the offset .epsilon. between the median plane 13 of rim 1 and plane 15 containing the anchoring spots 14 of spokes 11, 12.
However, the offset .epsilon. between the median plane 13 of the rim and the plane 15 where the spokes are applied gives origin to a drawback. As a matter of fact, when the wheel is mounted and the spokes are tensioned, the cross-section of the rim is subjected to a torsioning torque tending to bring the resulting force of the spoke tensions on the median plane of the rim, with a resulting rotation of the cross-section, along with a deformation. This condition is shown in FIGS. 3-5 of the annexed drawings. FIG. 3 shows the cross-section of the rim before the deformation which is caused by the torsioning torque T generated by the resulting force F applied by the spokes. FIG. 4 shows the rotated condition of the rim and FIG. 5 shows the rim cross-section at an enlarged scale and shows the deformed condition by dotted lines which is generated in the situation of FIG. 4.